Progression of atheroscierosis involves deposition of cholesteryl esters in macrophage muscle cells of the arterial wall. These cells become the foam cells characteristic of atherosclerotic plaque. Studies with cultured mouse peritoneal macrophages have shown that the cholesteryl esters in these cells are in a dynamic state, continually undergoing a cycle of hydrolysis and re-esterification. Other studies with cultured Fu5AH rat hepatoma cells have shown that cholesteryl ester clearance from these cells occurs more rapidly when the lipid is in a fluid, isotropic liquid state than when the lipid is more structured. These studies indicate that the steady state balance of the cholesteryl ester cycle depends on the physical state of the cholesteryl ester. We propose to develop model systems for studying kinetic and thermodynamic aspects of the phase behavior of cellular cholesteryl ester-rich inclusions characteristic of atherosclerotic plaque. These models will consist of phospholipid-stabilized dispersions of cholesteryl ester mixtures in phosphate buffered saline, pH 7.4. The dispersions will consist of droplets with average diameters varying from approximately 0.1 to 1 micron. Such dispersions are more appropriate for atherosclerosis-related research than bulk mixtures, which have been predominantly used in the past. This is particularly true for studying kinetic aspects of the phase behavior of these lipids. Initially, the core of the droplets will consist of binary mixtures of cholesteryl palmitate and cholesteryl oleate. Cholesteryl palmitate, with a relatively high melting point and enthalpy of fusion, promotes crystallization in these mixtures at physiological temperatures; cholesteryl oleate, with a lower melting point and smaller enthalpy of fusion, can exist in long-lived, metastable, liquid crystalline states at temperatures well below the melting point. The presence of cholesteryl oleate therefore promotes fluidity in these mixtures. Eventually, the composition of the droplet core will be varied as triglycerides, free cholesterol and other cholesteryl esters such as those containing poly-unsaturated fatty acyl groups will be included in the studies. Both egg phosphatidyl choline and dipalmitoylphosphatidyl choline will be used as dispersing agents. The equations of classical nucleation theory will be used to develop a theoretical model which will give the phase behavior of the dispersions as a function of time, temp- erature and lipid composition. These studies will be helpful in learning about the physical state of cellular cholesteryl estr-rich inclusions in living cells.